4-2
In this chapter, ac motors will be divided into (1) series, (2) synchronous, and (3) induction motors.
Single-phase and polyphase motors will be discussed. Synchronous motors, for purposes of this chapter,
may be considered as polyphase motors, of constant speed, whose rotors are energized with dc voltage.
Induction motors, single-phase or polyphase, whose rotors are energized by induction, are the most
commonly used ac motor. The series ac motor, in a sense, is a familiar type of motor. It is very similar to
the dc motor that was covered in chapter 2 and will serve as a bridge between the old and the new.
Q1.
What are the three basic types of ac motors?
SERIES AC MOTOR
A series ac motor is the same electrically as a dc series motor. Refer to figure 4-1 and use the left-
hand rule for the polarity of coils. You can see that the instantaneous magnetic polarities of the armature
and field oppose each other, and motor action results. Now, reverse the current by reversing the polarity
of the input. Note that the field magnetic polarity still opposes the armature magnetic polarity. This is
because the reversal effects both the armature and the field. The ac input causes these reversals to take
place continuously.
Figure 4-1.—Series ac motor.
The construction of the ac series motor differs slightly from the dc series motor. Special metals,
laminations, and windings are used. They reduce losses caused by eddy currents, hysteresis, and high
reactance. Dc power can be used to drive an ac series motor efficiently, but the opposite is not true.
The characteristics of a series ac motor are similar to those of a series dc motor. It is a varying-speed
machine. It has low speeds for large loads and high speeds for light loads. The starting torque is very
